Method and apparatus for detecting defects on a disk surface

ABSTRACT

The present invention relates to an apparatus for detecting defects on a disk surface which projects light on the disk surface by a light transmitting system, receives specula reflection light and scattered light by a light receiving system, exposes defects by performing a two-dimensional frequency filter process on a signal, and performs a defect determination process to extract a linear-shaped isolative defect candidate. Next, the present invention performs a periodicity determination process to classify and detect the periodically generated linear and circular arc defects and the isolatively generated linear and circular arc defects.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to detect defects on a disk surface, andmore particularly, to a method and apparatus for detecting lineardefects and circular arc defects that exist on a disk surface andperiodically occur.

2. Description of the Related Art

As a material of an information recording medium or a semiconductor, adisk such as glass, silicon wafer, and the like is used. When there aredefects on the surfaces of these materials, characteristics of a productare deteriorated. As a result, detection of these defects is performedby an apparatus for detecting defects on a disk surface. The apparatusfor detecting defects on the disk surface detects the defects generatedon the disk surface. There are various kinds of defects, for example,fine dust (particle) attached to the surface, spot (stain), abrasionmark (scratch) due to foreign materials, fine concave portions (pit) orconvex portions (bump), smoothly inclined concave portions (dimple),change (handling damage) of a disk edge caused due to collision, etc.,at the time of delivering a disk, a grinding mark (glide) of a disksurface, and the like. As a method for effectively detecting thesevarious defects, an apparatus for detecting defects on a disk surfaceaccording to the related art uses a method that detects defects byirradiating a laser beam on the disk surface and receiving each opticalcharacteristic differently detected in terms of shape, size and the likeof each defect, that is, reflected light or scattered light of the laserbeam. FIG. 20 shows a schematic configuration of the apparatus fordetecting defects on a disk surface that is disclosed inJP-A-2000-180376 and JP Patent No. 3732980 using the above-mentioneddetecting method.

In addition to the above-mentioned defects, a defect called ‘wrinkleshape’ (hereinafter, referred to as wrinkle defect) is formed on thedisk surface. The wrinkle defect is a defect that occurs during aprocess when the disk is shrunk by heat while the disk is manufactured.FIGS. 2A to 2E show features of the shape. FIG. 2A shows the whole diskand FIGS. 2B and 2C each show enlarged parts α and β where the wrinkledefect occurs. Further, FIG. 2D shows a cross sectional profile of FIG.2B, and FIG. 2E shows a cross sectional profile of FIG. 2C. The wrinkledefect, which is a defect that periodically generates rugged portions ina linear shape or a circular arc shape, is a low aspect defect of whicha height is very low as compared to the occurrence period of thedefects. If the above defect exists on the disk surface, it makes afloating amount of a head unstable and affects the accuracy of magneticreading and writing operations. As a result, this defect is consideredas a serious defect. Therefore, a disk having the wrinkle on its surfaceis considered as a defective disk and may be thus treated like defectivegoods.

The apparatus (FIG. 20) for detecting defects on a disk surface knownfrom the related art performs the detection for a low aspect (surfaceruggedness) defect by allowing a light receiving element 202 to receivespecula reflection light from a second light transmitting system 201.FIG. 3 illustrates the principle of detecting the low aspect defect. Thesecond light transmitting system 201 has a configuration to project aparallel light having a predetermined width on a surface of the disk 301that can detect the defects, and the light receiving element 202 has aconfiguration to receive the specula reflection light through a filter203 (FIG. 3A). When a concave defect 3011 exists on the surface of thedisk 301 (FIG. 3B), characteristics of the concave defect are similar tothose of a concave lens, such that parallel light can be collected inthe light receiving element 202. At this time, since a signal leveldetected by the light receiving element 202 is amplified, as shown inFIG. 3D as the amount of light received is amplified, it is possible todetect the concave defect by, for example, a threshold 2023. Further,when a convex detect 3012 exists on the surface of the disk 301 (FIG.3C), the convex defect is operated like a convex lens, such thatparallel light can be diffused and collected in the light receivingelement 202. At this time, since the signal level detected by the lightreceiving element 202 is reduced and amplified, as shown in FIG. 3E asthe amount of light received is reduced, it is possible to detect theconvex defect by, for example, a threshold 2024.

However, since the increase and decrease of the signal are extremelysmall in the wrinkle defect portion, it is difficult to detect thewrinkle defect having extremely low aspect ratio among the low aspectdefects. When detecting the wrinkle defects using an apparatus fordetecting defects on a disk surface according to the related art, thereare two problems as follows. The first problem is that the apparatus fordetecting defects on a disk surface according to the related art detectsthe disk in a spiral shape as shown in FIG. 4 and performs a thresholdprocess using a one-dimensional signal as a signal process. In thisprocess, when the defects periodically exist in a radial direction (rdirection) like the defect A of FIG. 4A, if the defect is detected inthe spiral shape, a periodic change in a signal like a signal of FIG.4B, which is intersected in a width direction of the defect, isobtained. However, if the signal strength is extremely small, when thethreshold process is performed on the signal, the signal change can behidden in the defect portion A by the change in the signal strength dueto the waviness of the disk.

In order to avoid this, a band pass filter, which passes through only afrequency band in which the defects exist, is generally applied to thesignal, making it possible to remove the waviness and detect the defectscaused during the threshold process. However, when the defectsperiodically exist in a circumferential direction (θ direction) like thedefect B of FIG. 4A, if the defects are detected in the spiral shape,there is a case where the change in the signal in the defect portion Blike a signal of FIG. 4B that is intersected in a length direction ofthe defect is changed to be approximately the same as the period of thewaviness. In this case, it is difficult to detect the defects even byusing the filter process.

The second problem is that even if there is the signal strength signalin the wrinkle defect portion like the defect portion A, since thechange in the signal strength is extremely small, the defects may beoverlooked due to the setting of the threshold or a large amount offalse reports may be generated. Further, distinguishing between thekinds of defects having linear or circular arc features, even if theyare detected, are insufficient.

There is a problem of having a bad effect on a hard disk or a problem ofgenerating a large amount of false reports by overlooking the wrinkledefect such that it is very inconvenient as the apparatus for detectingdefects on a disk surface.

SUMMARY OF THE INVENTION

The present invention provides an apparatus for detecting defects on adisk surface that can accurately distinguish and detect defects withoutoverlooking the wrinkle defects.

In other words, there is provided a method for detecting defects on adisk surface of the present invention including: irradiating light onthe disk surface; detecting linear defects from light reflected on thedisk surface; determining periodicity on the detected linear defects;and classifying and detecting the isolatively generated linear defectsand the periodically generated linear defects based on the determinationresult of the periodicity.

Further, there is provided a method for detecting defects on a disksurface of the present invention including: irradiating light on thedisk surface; detecting circular arc defects from light reflected fromthe disk surface; determining periodicity on the detected circular arcdefects; and classifying and detecting the isolatively generatedcircular arc defects and the periodically generated circular arc defectsbased on the determination result of the periodicity.

Moreover, there is provided an apparatus for detecting defects on a disksurface of the present invention including: a projecting unit thatirradiates a laser beam on the disk surface to scan the disk surface; alight receiving unit that receives reflection light of the laser beamdue to defects existing on the disk surface; and a signal processingunit that detects defects from an output of the light receiving unit toperform determination for each kind of defect, wherein the signalprocessing unit detects linear defects from the output of the lightreceiving unit, determines periodicity on the detected linear defects,and classifies and detects the generated linear defects and theperiodically generated linear defects based on the determination resultof the periodicity.

In addition, the signal processing unit detects circular arc defectsfrom the output of the light receiving unit, determines periodicity onthe detected circular arc defects, and classifies and detects thegenerated circular arc defects and the periodically generated circulararc defects based on the determination result of the periodicity.

According to the present invention, the wrinkle defect, which is a lowaspect defect, can be detected while suppressing false reports but notoverlook the defects regardless of the generation direction of thewrinkle defect. Therefore, the present invention is advantage in thatthe quality of the detected disk is secured and in some cases, the yieldof the disk product can be increased, and the production efficiency ofthe disk can be increased.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic configuration of an apparatusfor detecting defects on a disk surface;

FIG. 2A is a diagram showing an entire surface of the disk;

FIG. 2B is an enlarged diagram of the wrinkle defect α shown in FIG. 2A;

FIG. 2C is an enlarged diagram of the wrinkle defect β shown in FIG. 2A;

FIG. 2D is a diagram showing a cross section profile of the disk atplaces where the wrinkle defect α shown in FIG. 2B occurs;

FIG. 2E is a diagram showing a cross section profile of the disk atplaces where the wrinkle defect β shown in FIG. 2C occurs;

FIG. 3A is a configuration to allow a light transmitting system toproject parallel light having a predetermined width on a disk surfacethat can detect defects and the light receiving element to receive thespecula reflection light through a filter;

FIG. 3B is a configuration to project parallel light having apredetermined width on the disk surface to allow the light receivingelement to receive the specula reflection light through a filter whenconcave defects exist on the disk surface;

FIG. 3C is a configuration to project parallel light having apredetermined width on the disk surface to allow the light receivingelement to receive the specula reflection light through a filter whenconvex defects exist on the disk surface;

FIG. 3D is a diagram showing a signal level that is detected by thelight receiving element when the concave defects exist on the disksurface shown in FIG. 3B;

FIG. 3E is a diagram showing a signal level that is detected by thelight receiving element when the convex defects exist on the disksurface shown in FIG. 3C;

FIG. 4A is a diagram showing a state where there are a periodic defect Ain a radial direction (r direction) on the disk surface and a periodicdefect B in a circumferential direction (θ direction);

FIG. 4B is a diagram showing a signal obtained by detecting the defect Aon the rotating disk surface;

FIG. 4C is a diagram showing a signal obtained by detecting the defect Bon the rotating disk surface;

FIG. 5A is an enlarged diagram showing the wrinkle defects α shown inFIG. 2A;

FIG. 5B is an enlarged diagram showing the wrinkle defects β shown inFIG. 2A;

FIG. 5C is an enlarged diagram showing the wrinkle defects α shown inFIG. 2A;

FIG. 5D is an enlarged diagram showing the wrinkle defects β shown inFIG. 2A;

FIG. 6 is a diagram showing another configuration example of the lighttransmitting system of the apparatus for detecting defects on a disksurface;

FIG. 7 is a flow chart showing a process that detects linear defectshaving periodicity in a first embodiment;

FIG. 8A is a polar coordinate image that indicates a θ direction pixelon a horizontal axis and an r direction pixel on a vertical axis in animage obtained by a continuous signal detected;

FIG. 8B is a rectangular coordinate image that transforms the polarcoordinate image of FIG. 8A from the polar coordinate image in an actualspace so as to obtain a disk-shaped image and an actual defect-shapedimage;

FIG. 8C is a diagram for explaining a bilinear interpolation thatcorrects a mismatch of the polar coordinate pixels corresponding to therectangular coordinate pixels;

FIG. 9A is a diagram showing a band pass filter that passes through onlya frequency band in which the wrinkle defects exist;

FIG. 9B shows an n×n digital filter as an example of the band passfilter;

FIG. 10A is shows an image of an actual space (x and y) when extractionof a straight component of a binarized image is performed;

FIG. 10B is a diagram showing an image on a Hough space;

FIG. 11 is a diagram showing a length measuring result of the straightline detected by a Hough transformation;

FIG. 12A is a diagram showing an image on the disk surface;

FIG. 12B is an enlarged diagram of a place A portion where several linesdetected as a linear defect candidate exist in an image on the disksurface;

FIG. 13A is a diagram showing a state that sets search regions Q₁ to Q₂orthogonal to lines, including each point from a starting point P_(s) toan ending point P_(e) of the line;

FIG. 13B is a diagram showing a concentration value from a point Q₁ to apoint Q₂ in each search region by segmenting the search regions Q₁ to Q₂into three regions of m₁, m₂, and m₃;

FIG. 13C is a diagram showing a relationship between a defect contrastvalue and a height measuring value due to an actual profile;

FIG. 13D is a diagram showing a state that sets the search regions Q₁ toQ₂ for each point from a starting point to an ending point of the linesegment in a drawing for explaining a method that obtains a height ofthe line segment determined as periodicity defects;

FIG. 13E is a diagram showing a distribution of the concentration valuefrom a point Q₁ to a point Q₂ in the search region;

FIG. 14 is a flow chart showing a process that detects periodicitycircular arc defects according to a second embodiment;

FIG. 15A is a diagram showing a case where three points have a circlewith the same radius r in the drawing for explaining a principle of thecircular arc detection by the Hough transformation;

FIG. 15B is a diagram showing a state plotting information on thecircular arc in a space that is made by a center coordinate (a and b)and a radius of a circular arc;

FIG. 16 is a diagram for explaining a method that performs thedetermination on the circular arc defects detected by the Houghtransformation;

FIG. 17A is a diagram showing an image on a disk surface in a case whereseveral circular arcs determined as the circular arc defects exists in aregion A;

FIG. 17B is an enlarged diagram of the region A;

FIG. 18A is a diagram showing the circular arc defects determined asisolative defects;

FIG. 18B is a diagram showing a distribution of the concentration valuefrom a point Q₁ to a point Q₂ of the search region for each point from astarting point P_(s) to an ending point P_(e) of the circular arc;

FIG. 18C is a diagram showing a relationship between a defect contrastvalue of the circular arc defects determined as the isolative defectsand a height measuring value due to an actual profile;

FIG. 18D is a diagram showing a state that sets the search regions Q₁ toQ₂ for each point from a starting point to an ending point of the linesegment in a drawing for explaining a method that obtains a height ofthe circular arc defect determined as periodicity defects;

FIG. 18E is a diagram showing a distribution of the concentration valuefrom a point Q₁ to a point Q₂ in the search region;

FIG. 19A is a diagram showing a screen that inputs a linear defectclassification parameter and a screen that outputs a linear defectdetection result;

FIG. 19B is a diagram showing a screen that inputs a circular arc defectclassification parameter and a screen that outputs a circular arc defectdetection result; and

FIG. 20 is a schematic configuration diagram of an apparatus fordetecting defects on a disk surface of the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments will be described with reference to theaccompanying drawings.

First Embodiment

FIG. 1 is a diagram schematically showing an entire configuration of anapparatus for detecting defects on a disk surface according to a firstembodiment. As shown in FIG. 1, the apparatus for detecting defects on adisk surface includes dual optical systems 100 and 200 that are eachconfigured with a light transmitting system and a light receivingsystem. For example, the first optical system 100 detects pit, handlingdamage, stain, particle, and scratch defects and the second opticalsystem 200 detects bump, dimple, and glide defects. In other words, theplural optical systems are disposed according to the kind of defect. Thefirst light transmitting system 101 projects light to form laser spotson a surface of a disk 301, and the second light transmitting system 201projects parallel light having a predetermined width, which can detectdefects, on the surface of the disk 301 and at the same time, scans theposition thereof on the surface of the disk 301 in a spiral shape at astage 302. When the defects exist on the surface of the disk 301, thelaser spot is scattered, and therefore the plural defect data signalsare obtained by allowing each light receiving system to receive thescattered light.

In other words, the first light receiving element 102 receives a brightview field portion of the scattered light from the first lighttransmitting system 101 (that is, laser spot), and second lightreceiving elements 104 and 105 receive a dark field portion of thescattered light (however, in this example, the light receiving element105 receives the dark field portion scattered in a low angle directionfrom the disk surface, and the light receiving element 104 receives thedark field portion scattered in a high angle direction). The lightreceiving element 103 receives specula reflection light from the firstlight transmitting system 101, and a light receiving element 202receives the specula reflection light from the second light transmittingsystem 201, respectively. When there are defects, the specula reflectionlight increases and decreases and is then received in each of the lightreceiving elements 103 and 104. Thus, each of the light receivingelements is disposed so that it corresponds to the specula reflectionlight or the scattered light having different light strength accordingto the kind of defects. In addition, in order to receive targeted light(that is, the specula reflection light or the scattered light) with highefficiency, elements such as filters 108, 109, and 203, a lens 106, andthe like are disposed.

Light received by each of the light receiving elements is transformedinto each defect data signal through an amplifier circuit 4041 and thelike, which is in turn input to a signal processing device 401 (4011).Next, the defect data signal is subjected to a coordinate transformationprocess from a polar coordinate to a rectangular coordinate and isstored in an address of a memory corresponding to a predetermined unitcell (for example, a micro rectangular cell that is formed as a microdistance Δr in a radial direction and a micro distance Δθ in acircumferential direction on the disk) on the disk surface (4012).Thereafter, a filter process is performed on a coordinate transformedimage (4013). Further, the defect determination is performed from ashape feature of the defect such as the continuity and density of thestored address and the like (4015), and determination on periodicity fora line segment determined as the linear defect or the circular arcdefect in the defect determination is performed (4016). What is detectedas the defect (4018) is output from an output device 403.

The apparatus for detecting defects on a disk surface is configured ofthe optical system 100 and the optical system 200 that includes thelight transmitting system and the light receiving system, respectivelyand is disposed at a predetermined position so that the lighttransmitting system and the light receiving system can detect pluraldefects on the surface of the disk 301. In the first embodiment, each ofthe “pit”, “handling damage”, “stain”, “particle”, and “scratch” defectsis detected by the first optical system 100, and each of the “bump”,“dimple”, and “glide” defects is detected by the second optical system200. The wrinkle defect is detected by the second optical system 200.

The detection of the wrinkle defect is performed by allowing the lightreceiving element 202 to receive the specula reflection light from thesecond light transmitting system 201. The result obtained by performingthe detection on the wrinkle defect shown in FIGS. 2A to 2E uses theoptical system shown in FIGS. 5A to 5D. Comparing FIGS. 2A to 2E withFIGS. 5A to 5D, the ruggedness of the height and the ruggedness of thesignal strength are inverted to each other. This is because a concavedefect increases signal strength and a convex defect decreases in signalstrength, as shown in the detection principle of FIGS. 3A to 3E.However, the features, which periodically generate the linear defect orthe circular arc defect, do not change. Moreover, although the detectionprinciple using the parallel light as the light transmitting system 201in FIGS. 3A to 3E with regard to the optical system 200 is described, itis possible to detect the concave and convex defects on the surface byusing converged light shown in FIG. 6 as the light transmitting systemsimilarly to the above detection principle using the parallel light.

FIG. 7 is a flow chart showing one example of a program that performsthe determination on the kind of defects executed in the signalprocessing device 401 of FIG. 1. The processing flow chart is a flowchart that performs determination on the wrinkle defect that occurs in alinear shape. The flow chart determining the wrinkle defect that occursin the circular arc shape will be described later. The process starts byinputting the defect data signal from the light receiving element 202that detects the wrinkle defect, that is, allowing the light receivingelement 202 at a hardware side (that is, an optical system side) toreceive the signal strength change that is generated by the wrinkledefect (4011).

When a signal through a circuit (an amplifier 4041, and the like) fromthe light receiving element 202 at a software side (that is, a signalprocessing device 401 side) is input (4011), the coordinatetransformation process from the polar coordinate to the rectangularcoordinate is performed (4012). FIGS. 8A to 8C show the coordinatetransformation process of the acquisition signal. Since the detectionapparatus uses a spiral scanning method as shown in FIGS. 4A to 4C, theimage acquired by the detected continuous signal becomes the polarcoordinate image that takes a θ direction pixel at a horizontal axis andan r direction pixel at a vertical axis as shown in FIG. 8A. In order toobtain the disk shape image and the actual defect shape image in theactual space from the polar coordinate image, it needs the coordinatethe transformation from the polar coordinate to the rectangularcoordinate (FIG. 8B). The transformation from the pixels (x and y) amongthe polar coordinate images to the pixels (u and v) among therectangular coordinate images uses the following equation.

$\begin{matrix}{{u = {\left\{ {{\frac{R_{2} - R_{1}}{X}(x)} + R_{1}} \right\}{\sin\left( {2\;\pi\frac{y}{Y}} \right)}}}{v = {\left\{ {{\frac{R_{2} - R_{1}}{X}(x)} + R_{1}} \right\}{\cos\left( {2\;\pi\frac{y}{Y}} \right)}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

wherein R₁ indicates an inner circumferential radius of the disk, R₂indicates an outer circumferential radius of the disk, and X and Yindicate the number of the r direction pixels and the number of the θdirection pixels of the polar coordinate image, respectively. Herein,since each pixel (observation point) has a discrete value, the polarcoordinate pixel (x and y) corresponding to the rectangular coordinatepixel (u and v) do not match agree with the observation point. Themismatch is corrected by the bilinear interpolation at the adjacentobservation point. The bilinear interpolation performs the interpolationby the following equation using image data of four observation pointsaround an interpolation point as shown in FIG. 8C.P _((xy))={(i+1)−x}{(j+1)−y}P _(ij)+{(i+1)−x}{y−j}P _(ij+1)+{x−i}{(j+1)−y}P _(i+1j) +{x−i}{y−j}P _(i+1j+1)  [Equation 2]

wherein i and j are each the integer parts (observation points) of x andy.

Next, the filter process is performed on the coordinate transformedimage (4013). Since the wrinkle defect is a low aspect defect and thesignal strength is extremely small, when the threshold process isperformed on the signal, there is the possibility of burying the signalchange in the defect portion according to the signal strength change dueto the waviness of the disk or the noise of the high frequency device.In order to avoid this, as shown in FIG. 9A, a band pass filter, whichpasses through only a frequency band in which the wrinkle defects exist,is applied to the signal, making it possible to remove the waviness andnoise and detect the defects caused due to the threshold process.Herein, the frequency band is obtained from a period parameter of thedefect that is set by a user (or previously set to the device). Anexample of the filter is shown in FIG. 9B. FIG. 9B show an n×n digitalfilter, wherein the filter process is performed on the image byconvoluting the filter. The size and coefficient of the filter aredetermined so that the filter has a characteristic of passing throughonly the frequency band in which the wrinkle defects exist. Further, thebinarization process on the image, which is subjected to the filterprocess, is compared with the threshold set by the user (or previouslyset to the device).

Next, the extraction of straight component of the image binarized by theprocess is performed (4014). As an example for performing the straightdetection, a Hough transform method is used. FIGS. 10A and 10B show aprinciple of the Hough transformation. FIG. 10A shows an image on theactual space (x and y) and FIG. 10B shows the Hough space. For example,if three points, that is, P₁, P₂, and P₃ exist on a straight line, theequation of a straight line becomes the following equation.ρ=i x cos θ+y sin θ  [Equation 3]

wherein ρ is a perpendicular length lowered along a straight line fromthe left corner of the image and θ is an angle perpendicular to the xaxis. At this time, one straight line is equivalent to one point on aθ−ρ parameter space shown in FIG. 10B. Each point (P₁, P₂, and P₃)having a value of 1 in a binary image, which becomes an object, computesa pair of (θ and ρ) satisfying the above equation and plots the point asL_(p1) to L_(p3) of the FIG. 10B. L_(p1) to L_(p3) are intersected atonly one point (θ_(m) and ρ_(m)) and the point indicates a straight lineformed by P₁, P₂, and P₃ of FIG. 10A. In other words, if a point (thepoints θ_(m) and ρ_(m) of the greatest frequency) where the frequenciesmost commonly intersect are generated and selected on the θ−ρ parameterspace, the straight line where the (θ_(m) and ρ_(m)) is determined isthe most predominant straight component on the object image. Theextraction of the straight component is performed based on the aboveprinciple.

Next, the linear defect determination is performed on the pluralextracted line segments in respects to the distance between thecomposition points and the line segment length (4015). FIG. 11 shows thedetermination method. In FIG. 11, the straight line for the pointsselected on the θ−ρ parameter space is drawn on the x and y spaces, thethreshold parameter W having a width set by the user (or previously setto the device) for the straight line is taken, and a group of pointsexisting in the range becomes a group of points forming the straightline. The reason why the threshold parameter has the width is that theactual linear defect becomes a line segment having a predetermined widthwithout being overlapped with one line. Next, a distance g_(i) (i=1, 2 .. . ) between the group of points is obtained, and if the distance issmaller than the threshold parameter g_(t) of the distance set by theuser (or previously set to the device), the group of points isconsidered to be continuous and if it is larger than the thresholdparameter g_(t), it is considered to be separated. Next, the distancel_(i) (i=1, 2 . . . ) (a line segment length) between points of bothends for the group of connected points is obtained, and if the distanceis smaller than the threshold parameter l_(t) of the length set by theuser (or previously set to the device), it is considered that the groupof points do not have defects and if distance is larger than thethreshold parameter l_(t), it is considered that the group of points hasdefects. As shown in detail in FIG. 11, if intervals g₁ and g₂ becomeg₁<g_(t)<g₂, l₁ and l₂ become one line segment and l₃ becomes anindependent line segment. Further, if the line L, l₃, and l_(t) becomel₃<l_(t)<L, L is finally detected as a defect.

Next, the periodicity determination on the line segment determined asthe linear defect is performed (4016). FIGS. 12A and 12B show thedetermination method. FIG. 12B is an enlarged diagram of a portion ofplace A where several line segments determined as a linear defect existin an image on the disk of FIG. 12A. Herein, the determination processon periodicity is performed using the line segment L1 as a reference ofthe periodicity determination (4016). First, the directionalitydetermination is performed by the directionality threshold parameter Δθset by the user (or previously set to the device). In other words, withregard to an angle θ of a straight line obtained at the time ofperforming the Hough transformation, the line segment, which is withinthe range of θ₁±Δθ, is taken out as a candidate line segment havingperiodicity for the line segment L₁. For example, in the case of theFIG. 12B, it is determined that θ₂, θ₃, and θ₄ are matched with eachother in terms of directionality and θ₅ is not matched in terms ofdirectionality. Next, a region surrounded in a square shape from astarting point coordinate (x_(ls) and y_(ls)) of L₁, an ending pointcoordinate (x_(le) and y_(le)), and the threshold parameter ρ of theperiod set by the user (or previously set to the device) is considered,and it is determined that there are the line segment L1 and theperiodicity for the line segment, which is within the region. Forexample, in the case of FIG. 12B, the region is a region shown by adotted line and it is determined that L₂ included in the region hasperiodicity. Likewise, if L₂ is considered as a reference, it isdetermined that L₃ has periodicity and finally, it is determined thatL₁, L₂ and L₃ are a group of line segments having periodicity.

Determination on a height for the linear defect subjected to theperiodicity determination is finally performed (4017). Since theruggedness amount of the disk surface makes the floating amount of thehead unstable, it is necessary to perform the defect determinationdepending on the height. FIGS. 13A to 13E show an example of performingthe height determination from the image data. When a surface of a faceplate is examined using the optical system shown in FIG. 3, since theheight information appears as a contrast on the image, the heightdetermination is performed using the contrast information. In the linesegment determined as an isolative defect and the line segment havingperiodicity in the step of the periodicity determination, since a methodfor determining the height has a slight difference, the method thatobtains the height for the line segment determined as the isolativedefect will be first described referring to FIGS. 13A and 13B. Withregard to each point from a starting point P_(s) to an ending pointP_(e) of the line segment as shown in FIG. 13A, the search regions Q₁ toQ₂ orthogonal to the line segment including the points are set such thatthe contrast of the line segment is obtained from the concentrationvalue from the point Q₁ to the point Q₂ in the search region. Describingthis through FIG. 13B, it is considered that the search regions Q₁ to Q₂is divided into m₁, m₂, and m₃ and the line segment exists in the regionm₂. Since the contrast is compared with a circumferential portion, anaverage concentration at the portion is computed by the followingequation from portions m₁ and m₃ that are outside of the line segment.

$\begin{matrix}{C_{n} = {\sum\limits_{i \in D_{2}}^{\;}{{{\left\{ \frac{1}{m_{1} + m_{3}} \right\} \cdot {\sum\limits_{j \in {D_{1} + {D\; 3}}}^{\;}\left\{ f_{j} \right\}}} - f_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

A difference between the average concentration value and theconcentration value of the region m₂ is obtained, and the concentrationvalue obtained by integrating the difference in the region m₂ isobtained as the contrast at the point P_(n) in the line segment.

If a C_(n) value is computed from the starting point P_(s) to the endingpoint P_(e) of the line segment, subjected to the accumulation addition,and divided by the line segment length L, the average contrast C_(av) isobtained.

$\begin{matrix}{C_{av} = {\left( {\sum\limits_{P_{s}}^{P_{e}}C_{n}} \right)/L}} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

The defect determination can be performed from the contrast of thelinear defect that is detected by making the correlation between theheight measuring value according to the actual profile (measured byother devices) and the obtained contrast value into the shape as shownin FIG. 13C.

Next, the method for obtaining the height for the line segmentdetermined as the periodicity defect will be described using FIGS. 13Dand 13E. As shown in FIG. 13D, with regard to each point from thestarting point P_(s) to the ending point P_(e) of the line segment, thesearch regions Q₁ to Q₂ orthogonal to the line segment including thepoints are set such that the contrast of the line segment is obtainedfrom the concentration value from the point Q₁ to the point Q₂ in thesearch region. Describing this through FIG. 13E, since the surfaceruggedness continuously occurs in the periodic defect, it is consideredthat the level difference of the concentration continuously occurs. Theaverage concentration value of the search regions Q₁ to Q₂ is obtained,the absolute value of the difference between the average concentrationvalue and the concentration value of the search regions Q₁ to Q₂ isobtained, the obtained absolute value of the difference is integrated inthe search regions Q₁ to Q₂, and the value divided by the number of linesegments included in the regions Q₁ to Q₂ is obtained as the contrast atthe points P₁, P₂, and P₃ in the periodic line segment.

$\begin{matrix}{C_{n} = {\sum\limits_{i = Q_{1}}^{Q_{2}}{{{\sum\limits_{j = Q_{1}}^{Q_{2}}\left\{ f_{j} \right\}} - f_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

If the C_(n) value is computed from the starting point P_(s) to theending point P_(e) of the line segment, subjected to the accumulationaddition, and divided by the line segment length L, the average contrastC_(av) is obtained as follows.

$\begin{matrix}{C_{av} = {\left( {\sum\limits_{P_{s}}^{P_{e}}C_{n}} \right)/L}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

The defect determination can be performed on the basis of the contrastof the linear defect that is detected by making the correlation betweenthe height measuring value according to the actual profile (measured byother devices) and the obtained contrast value into the shape as shownin FIG. 13C.

As shown in FIG. 7, periodicity linear defect detection (4018),isolative linear defect detection (4020), and noise detection (4019) arefinally formed which are output from the output device 403 by theprocess. Further, the input of the defect classification parameters(4021) and (4022) is input by the user from the input screen of thedefect classification parameter input device 402, for example, as shownin FIG. 19A, and the output of the detection result is displayed by, forexample, separate colors for each kind of defect as displayed on theoutput screen 403.

Further, according to the first embodiment, the periodicity lineardefect, which is the low aspect defect, can be detected whilesuppressing false reports and not overlooking the defects regardless ofthe generation direction of the periodicity linear defect.

Second Embodiment

Next, the determination method of the wrinkle defect generated in thecircular arc shape will be described as the second embodiment. FIG. 14describes an image processing flow on the defect having the features ofthe circular arc distribution. The processing contents from 4011 to 4013of FIG. 14 are the same as the process described in FIG. 7.

Next, the extraction of circular arc component of the image binarized bythe process is performed (40141). As an example for performing thecircular arc detection, the Hough transform method is used. FIGS. 15Aand 15B show a principle of the Hough transformation. For example, inFIG. 15A, if three points, that is, P₁, P₂, and P₃ exist on the samecircle having a radius r, the circle equation becomes the followingequation.(x−a)²+(y−b)² =r ²  [Equation 8]

However, a and b become the center coordinate of a circle. The methodfor obtaining the center coordinate (a and b) indicates the circlehaving the radius r based on the points P₁, P₂, and P₃ and the pointthat most intersects the circle becomes the center coordinate (a and b).If the process is performed on ‘1’ pixel for the entire image, thecenter coordinate candidates of the circle whose radius is r areenumerated. The radius of the circular arc defect, which may be thedefect, is an arbitrary value, for example, the defect from the radiusr₁ to r₂ becomes an object range of the radius that is the fatal defect.In this case, the vote that chooses the point that most intersects thecircle is performed in the space (a_(m), b_(m), and r_(m)) to obtain theradius and center coordinates of the circular arc defect from theselected point.

Next, the determination of the circular arc defect is performed on theplural extracted circular arcs by the distance between the compositionpoints and the circular arc length (40151). FIG. 16 shows thedetermination method. First, a circle is drawn on an x and y plane fromthe value of (a_(m), b_(m), and r_(m)) obtained from the a, b, and rparameter space. The threshold parameter W having a width set by theuser (or previously set to the device) is obtained, and a group ofpoints existing in the range becomes the group of points forming thecircular arc. The reason why the threshold parameter has the width isthat the actual circular arc defect becomes a circular arc having apredetermined width without being overlapped with on one line segment.Next, a distance rθ_(gi) (i=1, 2 . . . ) between the group of points isobtained, and if the distance is smaller than the threshold parameterrθ_(gt) of the distance set by the user (or previously set to thedevice), the group of points is considered to be continuous and if thedistance is larger than the threshold parameter rθ_(gt), it isconsidered to be separate. Next, the distance rθ_(li) (i=1, 2 . . . )(circular arc length) between points of both ends for the group ofconnected points is obtained, and if the distance is smaller than thethreshold parameter rθ_(lt) of the length set by the user (or previouslyset to the device), it is considered that the group of points does nothave defects and if the distance is larger than the threshold parameterrθ_(lt), it is considered that the group of points has defects, and thedefects are detected. As shown in detail in FIG. 16, if intervalsrθ_(g1) and rθ_(g2) become rθ_(g1)<rθ_(gt)<rθ_(g2), rθ₁₁ and rθ₁₂ becomeone circular arc rθ_(L), and rθ₁₃ becomes an independent circular arc.Further, if the circular arc rθ_(L), rθ₁₃, and rθ_(lt) becomerθ₁₃<rθ_(lt)<rθ_(L), rθ_(L) is finally detected as a defect.

Next, the periodicity determination on the circular arc determined asthe circular arc defect is performed (40161). FIGS. 17A and 17B show thedetermination method. FIG. 17B is an enlarged diagram of a place Aportion where several circular arcs determined as the circular arcdefect exist in an image on the disk of FIG. 17A. Herein, thedetermination process on periodicity is performed using the circular arcC1 as a reference of the periodicity determination (40161). First, thesame centricity determination is performed by the threshold parameter(Δa and Δb) of the circular arc center coordinate set by the user (orpreviously set to the device). In other words, with regard to thecircular arc coordinate obtained at the time of performing the Houghtransformation, the circular arc, which is within the range of a±Δa andb±Δb, is selected as a candidate line segment having periodicity for theC1. For example, in the case of FIG. 17B, it is determined that (a₂ andb₂), (a₃ and b₃) and (a₄ and b₄) have the same center coordinate, while(a₅ and b₅) does not have the same center coordinate. Next, a regionsurrounded in a stripe shape from a starting point coordinate (x_(ls)and y_(ls)) an ending point coordinate (x_(le) and y_(le)) of C1, andthe threshold parameter r of the period set by the user (or previouslyset to the device) is considered and it is determined that there are thecircular arc C1 and the periodicity for the line segment, which iswithin the region. For example, in the case of FIG. 17B, the region is aregion shown by a dotted line and it is determined that C2 included inthe region has periodicity. Likewise, if C2 is considered as areference, it is determined that C3 has periodicity and finally, it isdetermined that C1, C2, and C3 are the group of circular arcs havingperiodicity.

Determination on a height for the linear defect subjected to theperiodicity determination is finally performed (40171). Since the convexand concave amount of the disk surface makes the floating amount of thehead unstable, it is necessary to perform the defect determinationdepending on the height. FIGS. 18A to 18E show an example of performingthe height determination from the image data. When a surface of a faceplate is examined using the optical system shown in FIG. 3, since theheight information appears as a contrast on the image, the heightdetermination is performed using the contrast information. In thecircular arc determined as the isolative defect and the circular archaving periodicity in the step of the periodicity determination, since amethod for determining the height is slightly different, the method thatobtains the height for the circular arc determined as the isolativedefect will be first described using FIGS. 18A and 18B. With regard toeach point from a starting point P_(s) to an ending point P_(e) of thecircular arc as shown in FIG. 18A, the search region Q₁ to Q₂ orthogonalto the vertical line to the focused points is set such that the contrastof the line segment is obtained from the concentration value from thepoint Q₁ to the point Q₂ in the search region. Describing this throughFIG. 18B, it is considered that the search region Q₁ to Q₂ is dividedinto three regions m₁, m₂, and m₃ and the circular arc exists in theregion m₂. Since the contrast is compared with a circumferentialportion, an average concentration at the portion is to be computed fromportions m₁ and m₃ that are outside of the circular arc. A differencebetween the average concentration value and the concentration value ofthe region m₂ is obtained, and the concentration value obtained byintegrating the difference in the region m₂ is obtained as the contrastat the point P_(n) in the line segment.

$\begin{matrix}{C_{n} = {\sum\limits_{i \in D_{2}}^{\;}{{{\left\{ \frac{1}{m_{1} + m_{3}} \right\} \cdot {\sum\limits_{j \in {D_{1} + {D\; 3}}}^{\;}\left\{ f_{j} \right\}}} - f_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

If a C_(n) value is computed from the starting point P_(s) to the endingpoint P_(e) of the circular arc, subjected to the accumulation addition,and divided by the circular arc length rθ_(L), the average contrastC_(av) is obtained as follows (Equation 10).

$\begin{matrix}{C_{av} = {{\left( {\sum\limits_{P_{s}}^{P_{e}}C_{n}} \right)/r}\;\theta_{L}}} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The defect determination can be performed from the contrast of thecircular arc defect that is detected by making the correlation betweenthe height measuring value according to the actual profile (measured byother devices) and the obtained contrast value into the shape, as shownin FIG. 18C.

Next, the method for obtaining the height for the circular arcdetermined as the periodicity defect will be described using FIGS. 18Dand 18E. As shown in FIG. 18D, with regard to each point from thestarting point P_(s) to the ending point P_(e) of the circular arc, thesearch regions Q₁ to Q₂ orthogonal to the vertical line to the focusedpoint are set such that the contrast of the line segment is obtainedfrom the concentration value from the point Q₁ to the point Q₂ in thesearch region. Describing this through FIG. 18E, since the surfaceruggedness continuously occurs in the periodic defect portion, it isconsidered that the level difference of the concentration continuouslyoccurs. The average concentration value of the search regions Q₁ to Q₂is obtained, the absolute value of the difference between the averageconcentration value and the concentration value of the search regions Q₁to Q₂ is obtained, the obtained absolute value of the difference isintegrated in the search regions Q₁ to Q₂, and the value divided by thenumber of circular arcs included in the regions Q₁ to Q₂ is obtained asthe contrast at the points P₁, P₂, and P₃ in the periodic line segmentas follows (Equation 11).

$\begin{matrix}{C_{n} = {\sum\limits_{i = Q_{1}}^{Q_{2}}{{{\sum\limits_{j = Q_{1}}^{Q_{2}}\left\{ f_{j} \right\}} - f_{i}}}}} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack\end{matrix}$

If the C_(n) value is computed from the starting point P_(s) to theending point P_(e) of the circular arc, subjected to the accumulationaddition, and divided by the circular arc length rθ_(L), the averagecontrast C_(av) is obtained as follows (Equation 12).

$\begin{matrix}{C_{av} = {{\left( {\sum\limits_{P_{s}}^{P_{e}}C_{n}} \right)/r}\;\theta_{L}}} & \left\lbrack {{Equation}\mspace{14mu} 12} \right\rbrack\end{matrix}$

The defect determination can be performed from the contrast of thelinear defect that is detected by making the correlation between theheight measuring value according to the actual profile (measured byother devices) and the obtained contrast value into the shape, as shownin FIG. 18C.

Periodicity circular arc defect detection (40181), isolative circulararc defect detection (40201), and noise detection (40191) are finallyformed which are output from the output device 403 by the process.Further, the input of the defect classification parameters (40211) and(40221) is input by the user from the input screen of the defectclassification parameter input device 402 as shown in FIG. 19B, and theoutput of the detection result is displayed by, for example, separatecolors for each kind of defect as displayed on the output screen 403.

According to the second embodiment, the periodicity circular arc defect,which is the low aspect defect, can be detected while suppressing falsereports and not overlooking the defects regardless of the generationdirection of the periodicity circular arc defect.

Third Embodiment

Although the first embodiment relates to the detection of theperiodicity linear defect and the second embodiment relates to thedetection of the periodicity circular arc defect, it is preferable tosimultaneously perform detection of these defects in the same apparatus.In this case, in the flow chart of FIG. 7, it is preferable that afterthe straight line detection step (4014) and the linear defectdetermination step (4015) or simultaneously with the above steps, thecircular arc detection step (40141) and the circular arc defectdetermination step (40161) shown in FIG. 14 are performed. Thereby, itis possible to detect the periodicity linear defect and the periodicitycircular arc defect, which are the wrinkle defect of the disk.

Fourth Embodiment

Although the above embodiments describe the processing flow for the lowaspect defect, the method for detecting defects can be applied to thedetection of the linear defect and the circular arc defect, which occuron the disk surface, for example, the detection of the scratch, etc. Inthis case, the process starts by allowing the light receiving elements102, 104, and 105 to receive the scattered light from the first lighttransmitting system 101 of the first optical system and to input thesignal to the signal processing device 401. In this case, the defectclassification parameter 402 is input by the user according to the kindof detected defects. By this process, it is possible to classify anddetect the periodically generated scratch and the isolatively generatedscratch.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

1. A method for detecting defects on a disk surface comprising:irradiating light on the disk surface; detecting linear defects fromlight reflected on the disk surface; determining periodicity on thedetected linear defects; and classifying and detecting the isolativelygenerated linear defects and the periodically generated linear defectsbased on the determination result of the periodicity.
 2. The method fordetecting defects on a disk surface according to claim 1, wherein theclassifying and detecting the isolatively generated linear defects andthe periodically generated linear defects performs determination onperiodicity for the detected linear defect using a generation coordinateof defect, a generation direction of a line segment, and a periodbetween line segments.
 3. A method for detecting defects on a disksurface comprising: irradiating light on the disk surface; detectingcircular arc defects from light reflected on the disk surface;determining periodicity on the detected circular arc defects; andclassifying and detecting the isolatively generated circular arc defectsand the periodically generated circular arc defects based on thedetermination result of the periodicity.
 4. The method for detectingdefects on a disk surface according to claim 3, wherein the classifyingand detecting the isolatively generated circular arc defects and theperiodically generated circular arc defects performs determination onthe periodicity of the detected circular arc defect using a generationcoordinate of the defect, a circular arc center coordinate, and a periodbetween the circular arcs.
 5. A method for detecting defects on a disksurface comprising: a first step of irradiating a laser beam on the disksurface; a second step of forming a disk image from light reflected onthe disk surface; a third step of performing emphasis on the disk imageusing a filter passing through only a frequency band where lineardefects exist; a fourth step of extracting straight components when thedefects for the image subjected to the emphasis occur in a linear shape;a fifth step of detecting the linear defect by performing a lengthmeasuring process on the extracted straight components; a sixth step ofperforming periodicity determination on the detected linear defect; anda seventh step of performing height determination when it is determinedin the sixth step that there is periodicity; wherein, when it isdetermined in the seventh step that there is a height having apredetermined value or more, it is determined that there is theperiodicity linear defect.
 6. The method for detecting defects on a disksurface according to claim 5, wherein the first step scans the disk in aspiral shape and the second step performs coordinate transformation on aone-dimensional array signal (polar coordinate) obtained from reflectionlight to perform the generation of the disk image (rectangularcoordinate).
 7. The method for detecting defects on a disk surfaceaccording to claim 5, wherein the third step exposes the defects for theobtained disk image generated in a radial direction (r direction) and acircumferential direction (θ direction) regardless of directivity byusing a two-dimensional frequency filter.
 8. The method for detectingdefects on a disk surface according to claim 5, wherein the fourth stepextracts the straight components from the image whose defects areexposed when the defects occur in a linear shape.
 9. The method fordetecting defects on a disk surface according to claim 5, wherein thefifth step performs the length measuring process on the extractedstraight components using parameters such as a defect width, a defectlength, and the like.
 10. The method for detecting defects on a disksurface according to claim 5, wherein the sixth step performs theperiodicity determination on the detected linear defects usingparameters such as a generation direction of defect, an interval betweendefects, a generation coordinate, and the like.
 11. The method fordetecting defects on a disk surface according to claim 5, wherein theseventh step determines the height from a contrast of the disk image.12. A method for detecting defects on a disk surface comprising: a firststep of irradiating a laser beam on the disk surface; a second step offorming a disk image from light reflected on the disk surface; a thirdstep of performing emphasis on the disk image using a filter passingthrough only a frequency band where circular arc defects exist; a fourthstep of extracting circular arc components when the defects for theimage subjected to the emphasis occur in a circular arc shape; a fifthstep of detecting the circular arc defect by performing a lengthmeasuring process on the extracted circular arc components; a sixth stepof performing periodicity determination on the detected circular arcdefect; and a seventh step of performing height determination when it isdetermined in the sixth step that there is the periodicity, wherein,when it is determined in the seventh step that there is a height havinga predetermined value or more, it is determined that there is theperiodicity circular arc defect.
 13. The method for detecting defects ona disk surface according to claim 12, wherein the fourth step extractsthe circular arc components from the image whose defects are exposedwhen the defects occur in a circular arc shape.
 14. The method fordetecting defects on a disk surface according to claim 12, wherein thefifth step performs the length measuring process on the extractedcircular arc components using parameters such as a defect width, adefect length, and the like.
 15. The method for detecting defects on adisk surface according to claim 12, wherein the sixth step performs theperiodicity determination on the detected circular arc defects usingparameters such as a generation direction of defect, an interval betweendefects, a generation coordinate, and the like.
 16. The method fordetecting defects on a disk surface according to claim 12, wherein theseventh step determines the height from a contrast of the disk image.17. An apparatus for detecting defects on a disk surface comprising: aprojecting unit that irradiates a laser beam on the disk surface to scanthe disk surface; a light receiving unit that receives reflection lightof the laser beam due to defects existing on the disk surface; and asignal processing unit that detects defects from an output of the lightreceiving unit to perform determination for each kind of defect, whereinthe signal processing unit detects linear defects from the output of thelight receiving unit, determines periodicity on the detected lineardefects, and classifies and detects the isolatively generated lineardefects and the periodically generated linear defects based on thedetermination result of the periodicity.
 18. The apparatus for detectingdefects on a disk surface according to claim 17, wherein the signalprocessing unit detects circular arc defects from the output of thelight receiving unit, determines periodicity on the detected circulararc defects, and classifies and detects the isolatively generatedcircular arc defects and the periodically generated circular arc defectsbased on the determination result of the periodicity.
 19. The apparatusfor detecting defects on a disk surface according to claim 17, whereinthe signal processing unit performs determination on periodicity for thelinear defects detected from the output of the light receiving unitusing a generation coordinate of defect, a generation direction of aline segment, and a period between line segments.
 20. The apparatus fordetecting defects on a disk surface according to claim 17, wherein thesignal processing unit performs determination on periodicity for thecircular arc defects detected from the output of the light receivingunit using a generation coordinate of defect, a circular arc centercoordinate, and a period between circular arcs.